DNS Basics

This page is a hands-on intro to Kubernetes DNS: how names like service.namespace.svc.cluster.local resolve, what records CoreDNS serves, and how to test resolution from inside a pod.

Prerequisites

• A running cluster on Kubernetes 1.36.2 with CoreDNS installed (the default cluster DNS).
• kubectl configured against the cluster.
• A debug image with DNS tools. nicolaka/netshoot bundles dig, nslookup, and nc.

Quick one-off debug pod:

kubectl run tmp --rm -it --image=nicolaka/netshoot --restart=Never -- bash

Basic Examples

1. Resolve a Service by short name

Inside the same namespace, the Service name alone is enough.

dig +short payments

• The pod's resolv.conf search list appends namespace.svc.cluster.local.
• So payments expands to payments.<namespace>.svc.cluster.local.
• The answer is the Service's stable ClusterIP.
• This only works from a pod in the same namespace as the Service.

2. Resolve across namespaces

Add the namespace label to reach a Service elsewhere.

dig +short payments.billing

• payments.billing expands via the svc.cluster.local suffix in the search list.
• Use this form whenever caller and callee live in different namespaces.
• No NetworkPolicy is bypassed; DNS resolution and traffic policy are separate concerns.

3. Use the fully qualified name

The absolute FQDN skips the search-list guesswork.

dig +short payments.billing.svc.cluster.local

• The trailing zone svc.cluster.local marks this as a Service A record.
• FQDNs avoid extra failed lookups from the ndots:5 search behavior.
• Prefer FQDNs in config files shared across namespaces or languages.

4. Inspect a pod's resolver config

See exactly what the kubelet injected.

kubectl exec tmp -- cat /etc/resolv.conf

search myns.svc.cluster.local svc.cluster.local cluster.local
nameserver 10.96.0.10
options ndots:5

• nameserver is the cluster DNS Service ClusterIP (often 10.96.0.10).
• The search domains enable short-name resolution.
• ndots:5 means names with fewer than 5 dots try the search list first.

5. Look up the DNS Service itself

CoreDNS sits behind a Service named kube-dns.

kubectl -n kube-system get svc kube-dns

• The Service is named kube-dns for backward compatibility even though CoreDNS serves it.
• Its ClusterIP matches the nameserver in every pod's resolv.conf.
• The backing pods are the CoreDNS Deployment in kube-system.

6. Resolve a Pod's SRV record for a named port

Services publish SRV records for named ports.

dig +short SRV _http._tcp.payments.billing.svc.cluster.local

• The format is _port-name._protocol.service.namespace.svc.cluster.local.
• SRV records return the port number and target hostname.
• Clients that support SRV can discover ports without hardcoding them.

7. Resolve an external name

CoreDNS forwards names it does not own to upstream resolvers.

dig +short api.github.com

• Names outside cluster.local are forwarded via CoreDNS's forward plugin.
• By default the upstream is the node's /etc/resolv.conf.
• This is how in-cluster pods reach the public internet by name.

Intermediate Examples

8. Distinguish ClusterIP from headless resolution

Headless Services return pod IPs instead of one virtual IP.

apiVersion: v1
kind: Service
metadata:
  name: cassandra
  namespace: data
spec:
  clusterIP: None
  selector:
    app: cassandra
  ports:
    - port: 9042

dig +short cassandra.data.svc.cluster.local

• clusterIP: None makes this a headless Service.
• The lookup returns one A record per Ready pod, not a single ClusterIP.
• StatefulSets use this to give each pod a stable name like cassandra-0.cassandra.data.svc.cluster.local.

9. Tune ndots to reduce lookup overhead

High ndots can cause several failed lookups for external names.

apiVersion: v1
kind: Pod
metadata:
  name: web
spec:
  dnsConfig:
    options:
      - name: ndots
        value: "2"
  containers:
    - name: web
      image: registry.example.com/web:1.4.0@sha256:abc123

• Lowering ndots makes external FQDNs resolve on the first try.
• Set dnsConfig per pod or use a fully qualified name with a trailing dot.
• Do not lower it so far that in-namespace short names stop working.

10. Point a pod at custom resolvers

dnsPolicy: None gives you full control of resolv.conf.

spec:
  dnsPolicy: None
  dnsConfig:
    nameservers:
      - 10.96.0.10
    searches:
      - myns.svc.cluster.local
      - svc.cluster.local
    options:
      - name: ndots
        value: "5"

• dnsPolicy: None ignores cluster defaults and uses only your dnsConfig.
• Useful for pods that must query a specific DNS setup.
• Keep the cluster DNS ClusterIP in nameservers if the pod still needs Service resolution.

11. Allow DNS under a default-deny NetworkPolicy

A default-deny egress policy blocks DNS unless you allow port 53.

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-dns
  namespace: myns
spec:
  podSelector: {}
  policyTypes:
    - Egress
  egress:
    - to:
        - namespaceSelector:
            matchLabels:
              kubernetes.io/metadata.name: kube-system
      ports:
        - protocol: UDP
          port: 53
        - protocol: TCP
          port: 53

• Name resolution and traffic policy are independent; a deny-all egress rule silently breaks DNS.
• This policy permits egress to the kube-system namespace where CoreDNS runs.
• Allow both UDP and TCP on port 53, since large answers fall back to TCP.
• Apply this before tightening egress so pods can still resolve Service names.

12. Compare nslookup and dig output

Use nslookup when a container lacks dig.

kubectl exec tmp -- nslookup payments.billing.svc.cluster.local

Server:    10.96.0.10
Address:   10.96.0.10#53
 
Name:   payments.billing.svc.cluster.local
Address: 10.104.12.7

• Server confirms the cluster DNS ClusterIP is answering.
• The returned Address is the Service ClusterIP for a normal Service.
• If you get NXDOMAIN, check the namespace label and that the Service exists.

Stack versions: This page was written for Kubernetes 1.36.2, Docker Engine 29.6.1 (BuildKit default), containerd (CRI runtime on nodes), Helm 3, Compose v2, Argo CD (latest - verify at build), and Gateway API (GA - verify controller support at build).